1. Field of the Invention
The invention relates to a method for the real time measurement of bit wear during oilwell drilling.
2. Background Information
In T. Warren, "Factors Affecting Torque for a Roller Cone Bit," appearing in Jour. Pet. Tech. (September 1984), Volume 36, pages 1500-1508, a model was proposed for the torque of a roller cone bit. The model was derived from the theory of the rolling resistance of a wheel or cutter. For a pure rolling action, without bearing friction, the model shows that ##EQU1## where M is the time averaged torque required to rotate the bit under steady state conditions, R is the rate of penetration, N is the rotary speed of the bit, W is the axial force applied to the bit, and d is the bit diameter. a.sub.2 is a dimensionless constant that is determined by the bit geometry, and in principle, is independent of rock properties.
Soft formation bits have cones that are not true geometrical cones, and the axes of the cones are offset from the center of the bit. These two measures create a large degree of gouging and scraping in the cutting action of the bit. This effect is taken into account by adding another dimensionless bit constant, a.sub.1, to the model ##EQU2## In practice, the constant a.sub.1 includes the effect of bearing friction. This contributes less than 10% of the total bit torque under typical operating conditions.
Generally a.sub.1 has a much greater value for soft formation bits than for hard formation bits because of the longer teeth and the gouging action. a.sub.2 is generally greater for hard formation bits than for soft formation bits because hard formation bits drill by a rolling action that crushes and grinds the rock.
Warren confirmed the validity of the model (2) on both field and laboratory data and showed that it is insensitive to moderate changes in factors such as bit hydraulics, fluid type and formation type. This does not mean that rock properties do not affect torque, but rather than the effect of rock properties on bit torque is sufficiently accounted for by the inclusion of penetration per revolution, R/N, in the torque model.